X-ray and gamma-ray computer tomography

The physical relationships between conventional radiography, radiometric inspection methods and computer aided tomography (CT) are reviewed. CT has been successfully applied for medical diagnosis since the early 70's. The same method can also be used in nondestructive materials evaluation if some modifications concerning the radiation source and the detector system are introduced. The development of an industrial CT-scanner by BAM and its practical application for some typical nondestructive testing problems are described. General criteria for the image quality and the spatial resolution of the system are discussed on the basis of the appropriate mathematical relationships. In practice the detection sensitivity of an industrial CT-scanner for voids, flaws or cracks in the object under inspection is limited by the finite photon output of available radiation sources. Numerical relationships between the photon source strength, the scanning time, the thickness of the material and the maximum achievable density resolution are derived. (orig.)